Particle-blast cleaning apparatus are well known in the industry. While sandblasting equipment is widely used for many applications, it has been found that the utilization of particles which naturally sublimate can advantageously be utilized as the particulate media of such equipment to minimize adverse environmental facts and cleanup required following the cleaning activity. For example, U.S. Pat. No. 4,617,064, which issued to the present inventor Moore on Oct. 14, 1986, discloses a particle-blast cleaning apparatus utilizing carbon dioxide pellets and a high pressure carrier gas. The particular particle-blast apparatus described in the '064 patent includes a body which houses a rotary pellet transport mechanism to convey the carbon dioxide pellets from a gravity feed storage hopper to the high pressure carrier gas stream for application of the pellets to a discharge nozzle. In order to ensure that the high pressure gas does not leak into the rotary transport apparatus, a rather complex system of variable pressure gas seals is necessary.
While the apparatus and method described in the '064 reference can successfully be utilized to accomplish particle-blast cleaning, the structure and its function has some very important practical drawbacks. In particular, due to the requirement that the high pressure gas be prevented from leaking into the system at the receiving station, this apparatus requires a rather complex set of circular face seals for providing an airtight seal of the rotary apparatus as it is rotated about a central axis. In this regard, the rotary apparatus is fitted with a corresponding set of circular face seals, and means to establish a force on such seals which is proportional in magnitude to the pressure of the transport gas. In order to achieve and maintain this critical sealing function, the circular seals must remain substantially flat in order to remain in intimate, continuous contact with the surfaces to be sealed. Consequently, the sealing surfaces must withstand a relatively great amount of friction, with such friction being applied at varying rubbing velocities across the diameter of such circular seals. The rubbing velocity and friction differentials, of course, tend to wear the seals at correspondingly differing rates creating a relatively difficult seal maintenance problem. Additionally, it has been found that the seal surface becomes subjected to erosion in critical sealing areas adjacent the receiving station due to occasional shearing of the particulate media at the cavity/receiving station interface. Moreover, the uneven wearing pattern and relatively high friction involved in maintaining these seals has been found to compromise the flatness of such seals, and in particular tends to warp the circular sealing surfaces thereby tending to reduce the effectiveness thereof. Finally, it has been found that the necessary spacing of adjacent cavities within the rotary transport means results in a slight time delay between successive discharges of pellets therefrom, causing a somewhat non-uniform or pulsating discharge of the particulate media from the apparatus. Although it has been contemplated that additional rotary mechanisms might be added to attempt to obviate such pulsating particulate delivery, it appears that the manifolding and synchronizing requirements necessary to appropriately combine additional rotary mechanisms is relatively complex and would require inefficient duplication of other parts of the system. Maintenance problems would, of course, correspondingly be multiplied.
Consequently, despite the prior work done in this area, there remain problems of economically and reliably achieving and maintaining a proper seal between the particulate media transporting apparatus and the high pressure conveying gas required to discharge such particulate media. Additionally, prior art apparatus and processes fail to achieve a relatively uniform delivery of sublimable particulate media in an economical and relatively simple manner. Consequently, prior art structures and processes delivered a relatively inefficient system with rather high maintenance costs.